The invention relates to a device for reducing the power supply connection rating required for elevator installations with electric hoisting drives which has an energy storage unit for electrical energy, and to a method serving the same purpose.
Passenger and freight elevators are usually driven by electric motors. Various principles of transmitting the hoisting force to the elevator car are used. In one embodiment, a rotation motor acts directly, or via a gearbox, on a traction sheave which drives suspension ropes which support and move the elevator car at one end and a compensating weight at the other end. In another embodiment, a rotation motor drives a hydraulic pump which in essence actuates via a hydraulic fluid the piston rod(s) of one or more hydraulic cylinder(s) which drive the elevator car directly or via driving ropes. According to a further drive principle the elevator car, or its compensating weight connected to it by suspension ropes, is moved up and down by means of a linear motor. In modern elevator installations, regulation of the elevator car speed usually takes place by means of a regulated change in the frequency of the alternating current of the motor.
Common to all these drives is that during acceleration and braking phases the electric motors consume 2 to 4 times more electric power than when traveling at constant speed, that the driving power required differs greatly depending on the respective car load, and that the ratio between the daily running time of these electric motors and their at-rest time is generally very low, for example less than 10%.
The brief power peaks when starting and braking affect the dimensions, and therefore the costs, of power supply cables, transformers, EMC power supply filters, fuses, and switchgear. A further disadvantage of the power peaks described is that they can cause voltage fluctuations in the power supply, and thereby negatively affect the illuminating quality of lamps or the functioning of electronic devices. Futhermore, in many places the briefly occurring power peaks described result in higher recurring charges for connection to the power supply.
However, the dimensions of the components of the power supply connection, and some components of the power supply to the drive, as well as the size of the recurring consumption-related power supply connection charges, depend mainly on the power drawn during the relatively short running time of the elevator drive motor, even if the average power requirement is only a fraction of this quantity.
EP 0 645 338 B1 describes a device for elevator installations with an energy storage device whose operating principle is not described in greater detail. This energy storage device is continuously charged with direct current by a charging device which is permanently supplied from the power supply. When peak power is required, stored energy is fed to the drive system in addition to a limited component of energy which is taken directly from the power supply. By limiting the power drawn directly from the power supply to a value which lies below the power required by the drive for travel at constant speed, the energy consumption which occurs during the time of travel can also be distributed over the time during which the elevator is at rest, in that during the time of travel the accumulator supplies the difference in power and is charged again during the at-rest time. In this way, a value for the power supply connection rating can be achieved which is below the power required for travel at constant speed.
EP 0 645 338 B1 quotes the state of the art according to GB 2 139 831 and DE 3 743 660 in which energy storage devices based on accumulators are described. Neither in the description nor in the claims of EP 0 645 338 B1 is a reference to any other storage principle to be found. The energy storage device, which is not described in more detail, is fed by a charging device with a rapid charge mode and trickle charge mode as is usual for accumulators. In view of these facts, it is assumed that the energy storage device referred to in EP 0 645 338 B1 is an electrochemical accumulator (secondary element).
For use as the sole means of energy storage, electrochemical accumulators have a number of important disadvantages. Large peaks in power requirement can only be met by accumulators with extremely large dimensions whose service life, which is short anyway, is drastically reduced by frequent withdrawal of peak energy. The very limited value of charging current allowed for an accumulator also greatly restricts the frequency with which the need for peak power can be met. This limitation on the allowable value of charging current is also a serious obstacle to the recuperation of braking energy into an accumulator.
The object of the present invention is to create a device for reducing the power supply connection rating of elevator installations of the type described above which avoids the disadvantages stated. In particular, the device shall be capable of compensating high and frequent power peaks, have a long service life, and through its ability to absorb energy rapidly, be able to store recuperated braking energy temporarily as it occurs.
This objective is met by the present invention in the form of a device to reduce the power supply connection rating of elevator installations with electric drive systems having an energy storage unit for electric energy, and characterized in that this energy storage unit contains capacitors in the form of supercapacitors, and a method to reduce the power supply connection rating of elevator installations with electric drive systems, characterized in that electric energy is stored in an energy storage unit (11) which contains capacitors in the form of supercapacitors (13) and that before each elevator trip the energy required for the forthcoming trip is determined from the available information such as the load situation and trip destination, that a check is made whether the momentarily present energy content of the energy storage unit (11) together with the continuous supply from the power supply is sufficient for the trip, and that if necessary the start is delayed until the energy storage unit is sufficiently charged.
The invention is based on the idea of using innovative capacitors, so-called supercapacitors, instead of, or in combination with, accumulators as energy stores, the usual arrangement employed consisting of several supercapacitors connected in series, having a total capacitance of several farads at allowed voltages from 100 to 300 volts. Supercapacitors are double-layer capacitors whose electrodes consist of active carbon and therefore have effective surfaces of several thousand square meters per gram of carbon, the two electrodes being separated by minimal distances in the nanometer range. These characteristics result in the extremely high capacitance of these energy stores, which are commercially available from specialist suppliers.
The device according to the invention for reducing the power supply connection rating of elevator installations, where a large number of starting and braking operations cause high power peaks, has many advantages. By comparison with energy stores based on accumulators, supercapacitors have the following very positive characteristics:
Practically unlimited service life by comparison with accumulators.
High allowable charging and discharging power with a large number of charging and discharging cycles and low weight (power density of supercapacitors approx. 10-15 kW/kg; power density of accumulators approx. 300-1000 W/kg).
Complete charging and discharging is guaranteed even with high charging and discharging currents.
No maintenance is required.
They contain no toxic or environmentally harmful substances.
Low weight compared with accumulators having the same power density.
For applications in which the device according to the invention serves to compensate for power peaks and also, if necessary, to reduce the power supply connection rating by a relatively modest amount, it is expedient to use an energy storage unit which uses supercapacitors only as storage medium.
For applications in which the device according to the invention serves not only to compensate for power peaks but also to reduce the power supply connection rating of the elevator installation far below the energy required for travel at constant speed, it is advantageous to use an energy storage unit consisting of a combination of supercapacitors with electrochemically acting secondary elements (accumulators), since the latter, by comparison with supercapacitors, have a higher energy density (Wh/kg), i.e. have a higher storage capacity for the same weight. By suitably limiting the power drawn from the power supply, it can be so distributed over running time and at-rest time that the required power supply connection rating can be reduced to a fraction of the power required for travel at constant speed. This is made possible in that during phases in which the power of the motor is higher than the limited power drawn from the power supply the difference in power is fed from the energy storage unit, brief current peaks being taken mainly from the supercapacitors and longer constant power mainly from the accumulator, and that especially during at-rest times the energy storage unit is recharged.
According to a further embodiment of the invention, in elevator installations which have no power supply connection at all, or only one with a minimal power supply connection rating, a combination of an energy storage device made from supercapacitors and fuel cells, i.e. with electrochemically acting primary elements, can be used. In this case, the electric energy required for driving is wholly or partly generated in the fuel cells, whereas the supercapacitors serve as energy stores to cover power peaks and to distribute the power consumption over part of the elevator at-rest times.
A preferred embodiment of the device to reduce the power supply connection rating of elevator installations according to the invention acts in conjunction with one or more frequency converter(s). In each case one of these serves to regulate the rotational speed of one associated elevator drive motor. In essence, a frequency converter consists of a power converter, a voltage-controlled DC link with smoothing capacitor, and an inverter with control generator. In embodiments in which the power converter is not intended for recuperation of braking energy, the DC link is usually equipped with a braking module. The device according to the invention, which contains an energy storage unit of supercapacitors, or of a combination of these with an accumulator, takes energy (also braking energy) from the aforementioned voltage-controlled DC link and returns the energy to it during drive situations which require greater electric power than is supplied by the power converter with its limited current. In this case, a regulating and controlling unit known as a power-flow controller ensures adaptation as necessary of the DC voltage between the energy storage unit and the DC link, and regulates the exchange of energy between this energy storage unit and the DC link of the frequency converter.
An advantageous further development of the device according to the invention to reduce the power supply connection rating of elevator installations is achieved in that the power generated by the drive motor during braking phases, including the power peaks then generated, is recuperated into the supercapacitors of the energy storage unit. When doing so, if their charge capacity is exceeded, the power-flow controller stops the flow of energy into the energy storage unit, as a result of which the voltage in the DC link of the frequency converter increases until the braking module is activated, which uses a braking resistor to convert the excess electric braking energy into heat. By recuperating the braking energy into the energy storage unit, the costs for energy consumption can be effectively reduced, in contrast to recuperation into the power supply, which is usually not considered as a reduction in energy consumption.
In elevator installations in which one or more elevator vehicle(s) with integral drive system travel, it is advantageous to install the frequency converter, the elevator control unit, and the device according to the invention to reduce the power supply connection rating, in mobile fashion on the elevator vehicle(s). Each energy storage unit of the vehicles is then charged either via contact elements or by means of contactless energy transmission systems. As well as reducing the power supply connection rating required, this method has the advantage that devices for the supply of energy are not required along the entire length of travel. This is of particular interest in elevator installations where there are several elevator hoistways and the elevator vehicles change the elevator hoistways in which they travel, horizontal travel also taking place.
On elevator installations having several elevators adjacent to each other which are equipped with frequency-converter controlled drives, it is advantageous to connect the DC links of all the inverters on the motor side in parallel, and to supply them with power through a single power supply module. Connected to this common DC link is a single device to reduce the power supply connection rating of elevator installations according to the invention having an energy storage unit of supercapacitors, if necessary in combination with accumulators. As a result, energy compensating processes can take place directly between the individual motors, i.e. braking energy occurring momentarily from one or more drive motor(s) is used directly by one or more other momentarily driving drive motors(s). Further important advantages of this embodiment are that only one single energy storage unit with its control electronics, only one power supply module, and only one single braking module are required. In suitable cases, e.g. on groups with several elevators, and where energy recuperated into the power supply also pays for itself, it is expedient to recuperate excess braking energy into the power supply by means of a recuperation unit, in which case the braking module can be dispensed with.
On installations where the available power supply connection rating is less than the power required for travel at constant speed, i.e. where it is essential during such travel to supply additional energy from an energy storage unit, it is expedient to check the present energy supply situation before travel commences. The elevator control operates according to an energy management process which determines the energy required for the forthcoming trip based on available information about the load in the car and the trip destination, and then checks whether the energy content momentarily available in the energy storage unit together with the limited, continuous supply from the power supply, is sufficient for the trip. If necessary, the start is delayed until the energy storage unit is sufficiently charged.